This invention relates in general to data collection systems which use remotely located telemetry devices to transfer telemetry data from a remote site to a central processing location. More particularly, the invention relates to automatic meter reading (AMR) systems which use conventional subscriber telephone lines to transfer telemetry data, in the form of utility meter readings, from a customer's premises to a central processing location.
Prior to the existence of automatic meter reading (AMR) systems, the most common method for determining the amount of commodity delivered to a utility customer was to manually read a meter at, or in close proximity to, the consumer premises. Because the utility meters were located at the point where the utility commodity was dispensed to the customer, it became necessary for utility companies to establish routes where a "meter-reader" periodically visited each meter on the route to record the amount of utility product consumed.
At present, many utilities, including gas, electric and water companies, continue to send meter-readers to consumer residences' to collect utility meter readings. However, there are practical limitations as to how often and how efficiently this procedure can be manually performed. For instance, weather and the ability to gain access to meters themselves (which were often inside the consumer's residence) directly impact the efficiency of this manual procedure. Today, where it is desirable for the utility to have almost instantaneous access to any meter, the manual method for collecting these readings is becoming both economically and operationally obsolete in view of the more sophisticated and reliable automatic techniques now available.
One very practical method for automating the process of collecting utility meter readings utilizes the existing telephone system, thereby taking advantage of the already widespread availability of telephone service to both residential and business premises. Using this existing infrastructure, remotely located telemetry devices (at each consumer's premises) electronically upload utility meter readings as telemetry data to a central processing location via the subscriber telephone lines. This process is analogous to the procedure used by many PC users to electronically upload files by the use of a modem connected to the subscriber's telephone line, except that the AMR procedure is fully automatic. This invention relates to those AMR systems which utilize telephone line telemetry techniques.
In practice, the actual telemetry device is called a meter interface unit (MIU). Located at the customer premises, the MIU, as the name implies, is an interface between two different electrical environments. One side of the MIU, called the meter side or control side, is connected to one or more utility meters while the remaining side of the MIU is connected in parallel across the subscriber telephone line. In this regard, the telephone line side of the MIU is connected to the phone line in a manner identical to that used to add an additional extension phone, answering or FAX machine. Other than connecting-the MIU to the subscriber line, no modification of the existing telephone line wiring is required.
In one particular type of AMR system, a real time clock within the MIU activates the device at a prescribed date and time. Once activated, the MIU seizes,the phone line, dials a preprogrammed telephone number to connect with a central processing location, reads the utility meters connected to it and then uploads the telemetry data via the telephone line. Automatic meter reading systems which utilize this technique are known as dial-inbound systems, since the MIU dials into a data processing center.
Functionally, the MIU can be considered to be a "smart" telephone in that it automatically determines when the phone line is available for use, takes itself off-hook, dials a preprogrammed telephone number, communicates over the phone line, and then hangs up. Likewise, the telephone central office system cannot distinguish the MIU operation from a manual telephone call the subscriber might make.
Since the telemetry transaction typically takes only a few seconds to complete, the MIU normally resides in a low power, standby condition until such time as the programming of the MIU's real time clock causes the device to be activated again. Therefore, the MIU is said to be in a static mode between telemetry transactions and in a dynamic mode while engaged in the transfer of telemetry data across the telephone line.
Because it is not acceptable for the MIU to disrupt or otherwise interfere with the normal operation of subscriber telephone line, the MIU must be capable of detecting when the subscriber phone line is or is not in use. As noted before, the MIU is in either a static mode or a dynamic mode. As a result, the off-hook detector consists of not one function but two, namely a "dynamic off-hook detector" and a "static off-hook detector", where the terms dynamic and static coincide with the current mode of the MIU.
At first glance, the design of off-hook detectors may seem deceptively simple. However, getting them to function reliably, in practice, is a task requiring specialized design knowledge and skill. My patent, entitled Signal Processing Circuit For Use In Telemetry Devices, U.S. Pat. No. 5,202,916, the disclosure thereof being incorporated herein by reference, describes some of the complexities involved in designing a static off-hook detector for an MIU. Whereas the static off-hook detector serves to prevent the MIU from going to the dynamic (off-hook) mode while the subscriber line is in use, the dynamic off-hook detector permits the MIU to immediately disengage itself from the phone line should the subscriber attempt to use the telephone while the MIU is actively engaged in a telemetry transaction.
When a telephone device is taken off-hook, the impedance at the terminals of that device drops from a very high on-hook impedance to a few hundred ohms. With the normally open end of the subscriber line now terminated with this low impedance, the telephone set draws a loop current from the central office switch of approximately 40 mA. As long as loop current is being drawn, the central office switch sees the subscriber line as being off-hook. The telephone audio signal appears as a direct modulation of the loop current, as an AC signal superimposed on the DC loop current.
Many dynamic off-hook detectors function by sensing changes in the subscriber loop current to detect when contention for the subscriber line has occurred. For example, the invention described and claimed in my copending patent application entitled "Telemetry Device Including A Dynamic Off-Hook Detector", (Ser. No. 08/128,865) sets forth one such system. The disclosure of patent application "Telemetry Device Including A Dynamic Off-Hook Detector", (Ser. No. 08/128,865) is incorporated herein by reference. Basically, when another telephone device comes off-hook, it appears in parallel with the active MIU, thereby shunting current away from the telemetry device or "robbing current" therefrom. By detecting the "current robbing" effect caused on the MIU by the contending device, a dynamic off-hook detector can be implemented.
Another telemetry device including a dynamic off-hook detector which senses current robbing by the contending device to determine line status is described and claimed in my patent entitled "Outbound Telemetry Device", U.S. Pat. No. 5,204,896, the disclosure of which is incorporated herein by reference.
Although it is rapidly being supplanted by the DTMF dialing format (popularly called touch tone dialing), pulse dialing remains the only universally accepted dialing format. In fact, pulse dialing is still in wide use, especially in Europe. As a result, some conventional MIU devices do not even incorporate tone dialing and those which do often revert to the pulse dialing format if the MIU is not successful after one inbound dialing attempt.
The "pulse dialing" or rotary pulse format is based upon the mechanical rotation of the old-style telephone dial which interrupts the loop current in a prescribed manner. Because the make and break action of this rotary switch alternately connects and disconnects the telephone device, the formal designation for pulse dialing is "loop disconnect signalling." Both the break/make ratio (typically 60%) and the pulse rate (typically 10 pps) are specified parameters in the format.
Although the pulse dialing format is universal, it can take up to one second per digit to dial the designated telephone number. Consequently, since it requires several seconds to pulse dial, there is a high probability that if the telemetry session is interrupted, it will be while the MIU is pulse dialing. Ideally, should this occur, the MIU must surrender up the telephone line and disengage itself.
Again emphasizing that the term dynamic in dynamic off-hook detector refers to the mode of the MIU, a unique situation arises when the MIU attempts to use pulse dialing. Recalling that pulse dialing involves the MIU going on-hook and off-hook to pulse the loop current, the dynamic off-hook detector, which normally functions by sensing the current shunted away from the active MIU, has no current to measure while the MIU is in the on-hook portion of the dial pulse. Corresponding to the typical break/make ratio, the average loop current during pulse dialing would drop to 60% of its nominal value which the dynamic off-hook detector would interpret as another off-hook telephone device. Consequently, the MIU experiences current robbing during pulse dialing and would disengage itself from the telephone line shortly after pulse dialing began.
To avoid this undesirable effect, one approach is to disable the dynamic off-hook detector during pulse dialing. While this might adequately solve the immediate problem, it has the simultaneous disadvantage of disabling the dynamic off-hook detector function entirely while the MIU was pulse dialing. Consequently, if a telemetry session was interrupted while the telemetry device was pulse dialing, the contention for the subscriber telephone line will go undetected and the MIU will not release the phone line, as it should.